A basic form of the pressure measuring cell of the field of the invention is disclosed, for example, in European patent application EP 0351701 A2. It includes a measuring membrane and a platform, wherein the measuring membrane is pressure-tightly connected with a peripheral joint made from an active braze, wherein the capacitance between the first and the second electrodes is a measure for the difference between a pressure externally acting on the measuring membrane and a pressure reigning in the reference pressure chamber between the platform and the measuring membrane.
The joint can especially exhibit an active braze, with which a lastingly stable vacuum tight connection between the platform and measuring membrane is achieved.
An advantage of capacitive ceramic pressure measurement cells lies in the fact that they are “dry”, which means either ambient air or a vacuum is in the reference pressure chamber, so that, in the case of a breaking of the measuring membrane, which is directly exposed to the measured medium, no contamination of the measured medium via a pressure transfer liquid can occur, which is present in the case of pressure sensors equipped with a pressure transfer means.
Capacitive ceramic pressure measurement cells are usually installed in a housing of a pressure sensor, wherein an O-ring is axially clamped between the housing and the pressure measuring cell. In order to prevent reactions of the axial clamping on the deflection of the measuring membrane, the O-ring should lie on the measuring membrane in the region supported by the peripheral joint. Manufacturing tolerances and mounting tolerances of the O-ring lead to a minimum width for the joint, which, in the case of given outer dimensions of the platform or the pressure measuring cell, reduces the available area for the electrodes, which directly negatively affects the measured capacitance between the electrodes.
In the case of comparatively large measuring cells, this can be tolerable; in the case of more compact structures, however, a further reduction of the electrode surface leads to marked limitations, which is not readily acceptable.